Various labels have been attached to articles so that the articles can be distinguished one from the other. For example, bar code labels are attached to articles of grocery and are scanned at a check-out counter in order to automatically identify the articles and to register the price of the articles as they are purchased.
Bar code labels have also been used in inventory control and monitoring. Accordingly, these bar codes may be scanned in order to track articles as they move into, through, and out of a storage area. It is also known to read the bar codes attached to articles in order to access various computer records regarding the articles.
Bar code labels, however, have several drawbacks. For example, computer stored records that are accessed when a bar code is read do not move with the corresponding article. Therefore, if the article to which the bar code label is attached is remote from the computer, the records concerning that article cannot be immediately accessed if necessary.
Moreover, bar code labels cannot be read remotely. Thus, if it is desired to take an inventory of articles currently in the storage area, personnel must physically scan each label on each article one at a time in order to determine which articles are presently in the storage area. Such scanning requires the physical presence of the personnel at the location of the articles and is extremely time consuming. Additionally, because bar code labels cannot be read remotely, they cannot be used as security devices that can be detected if the articles to which they are attached are improperly removed from a secured area.
Instead of bar coded labels, it is known to attach radio frequency identification (RFID) tags to the articles to be monitored. The RFID tags can be read, as can bar code labels. However, unlike bar code labels, reading RFID tags does not require the physical presence of personnel because the RFID tags can instead be read remotely. Thus, inventory can be taken more quickly because personnel are not required to walk around a storage area or other area in order to read the RFID tags. Moreover, because RFID tags can be read remotely, they can be used as security devices. Thus, if someone attempts to surreptitiously remove an article to which an RFID tag is attached from a secured area, a remote reader can sense the tag and provide an appropriate alarm.
RFID tags can be read one at a time or in groups. When multiple RFID tags in a group are read at the same time, the information transmitted by the multiple tags frequently collide. Accordingly, spread spectrum techniques, such as either direct sequence spread spectrum (DSSS) or frequency hopping, in the communications between the reader and the tags have been suggested in order to reduce the impact of such collisions. It is also known to interrogate a tag using either a direct sequence spread spectrum (DSSS) signal or a frequency hopping signal.
In one embodiment, the present invention combines a direct sequence spread spectrum RF receiver and a frequency hopping transmitter in an RFID tag in order to overcome deficiencies of prior art tags. In another embodiment, the present invention relies on a communications protocol that supports the use of frequency hopping communications between the RFID tag and a reader.
The combination of direct sequence spread spectrum for signal reception and frequency hopping for signal transmission relative to a tag has not been previously suggested. Both of these modulation techniques circumvent jamming or interference by other signals. Also, the use of a direct sequence spread spectrum RF receiver in a tag permits the tag to properly synchronize to, and decode, the signal received from the reader in a shorter period of time than if the reader transmits the signal using frequency hopping. At the same time, this arrangement permits the tag to be interrogated by the reader over long distances while conserving power.